Radioisotope fuel material and method

ABSTRACT

A primary alpha-particle-emitting radioactive fuel material and a method for preparing it so that secondary neutron generation by an alpha-neutron reaction is substantially reduced. Those individual isotopes of oxygen, carbon, nitrogen, silicon, and chlorine which have a threshold for the alpha-neutron reaction greater than the maximum energy of the emitted alpha particles are selected for combination with the alpha-particle-emitting radioactive isotope to give the desired oxides, carbides, nitrides, silicides, and oxychlorides.

United tates Patent 1191 Kesltishian *E eb. 5, 1974 RADIOISOTOPE FUELMATERIAL AND METHOD [75] Inventor: Vahe Keshishian, Sherman Oaks,

Calif.

[73] Assignee: Rockwell International Corporation, El Segundo, Calif.

[ Notice: The portion of the term of this patent subsequent to June 2,1987, has been disclaimed.

[22] Filed: Jan. 13, 1971 211 Appl. No.: 106,071

Related US. Application Data [60] Continuation of Ser. No. 815,131,April 10, 1969, whichis a continuation-in-part of Ser. No. 687,945, Oct.25, 1967, which is a division of Ser. No. 464,702, June 17, 1965, Pat.No. 3,515,875.

[52] US. Cl 252/3011 R, 423/249, 423/250, 423/251, 423/252, 423/253,136/202 [51] Int. Cl C01g 56/00 [58] Field of Search 252/30l.1 R;23/343-347, 354-355; 136/202 [56] References Cited UNITED STATES PATENTS3,420,640 l/l969 Porter 23/344 OTHER PUBLICATIONS Argo, et a1.; NeutronEmission by Polonium Oxide Layers, Nuc. Sci. Abs., Vol. 10, No. 11, Abs.No. 3648, 1956, p. 464.

TerentEv, Polonium Oxides, Nuc. Sci. Abs., Vol. 10, No. 17, 1956, p.791.

Richter, Ceramic Tubes Developed For External Heat Sources, Nuc. Sci.Abs., Vol. 15, No. 18A, Abs. No. 23676, 1961 p. 3053.

Snap Programs, Nuc. Sci. Abs., Vol. 15, No. 10, 1961, Abs. No. 12652, p.1620.

Madorsky, et al., Concentration of Isotopes of C1 by l theCounter-Current Electromigration Method, J. of Research, N.B.S Vol. 38,1947, p. 185.

Kistemaker et al., Proc. of the lntemat. Symposium on IsotopeSeparation, 1958, pp. 158, 336, 337.

Calvin, et al., Isotopic Carbon, 1949, Wiley & Sons, p. 4.

Stable Isotopes, USAEC Publication, 1948.

Sheft, et al., Equilibrium in the Vapor-Phase Hydrolysis of PlutoniumTrichloride, The Transuranium Elemen eiisfitawlfl llt1242 39 i11:8f17-Keshishian et al., Use of O with P to Reduce Neutron Yield, Trans. AmNuc. Soc., Vol. 9, No. 1, 1966, p. 102.

Rutherford et al., Preparation of 0 Reduced in Masses 17 and 18, andEffecton Total Neutrons Emitted from PuO Trans. Am. Nuc. Soc., Vol. 9,No.2, 1966, p. 599-600.

McVey, Possible Requirements for Radioisotopes as Power Sources, Nuc.Sci. Abs., Vol. l5,'No. 21, 1961, Abs. No. 27914, p. 3600.

Primary Examiner-Carl D. Quarforth Assistant Examiner-R. L. TateAttorney, Agent, or Firm-L. Lee Humphries; Henry Kolin [57] ABSTRACT Aprimary alpha-particle-emitting radioactive fuel material and a methodfor preparing it so that secondary neutron generation by analpha-neutron reaction is substantially reduced. Those individualisotopes of oxygen, carbon, nitrogen, silicon, and chlorine which have athreshold for the alpha-neutron reaction greater than the maximum energyof the emitted alpha particles are selected for combination with thealpha-particle-emitting radioactive isotope to give the desired oxides,carbides, nitrides, silicides, and oxychlorides.

14 Claims, 1 Drawing Figure RADIOTSOTOPE FUEL MATERIAL AND METHOD CROSSREFERENCES TO RELATED APPLICATIONS This application is a continuation ofapplication Ser. No. 815,131, filed Apr. [0, 1969, which is acontinuation-in-part of application Ser. No. 687,945, filed Oct. 25,1967, which is a division of application Ser. No. 464,702, filed June17, 1965, now U. S. Pat. No. 3,515,875.

BACKGROUND OF THE INVENTION This invention relates to a radioactive fuelmaterial and to a method for preparing such radioactive fuel materialfor a radioisotope generator. More particularly it relates to analpha-particle-emitting fuel material for a radioisotope generatorwherein neutron shielding requirements are substantially reducedcompared with similar generators.

Radioisotope-powered generators are known. Such units are of particularinterest for space missions for supplying the power needed by theinstruments of the space vehicle. These generators are also of utilityin situations where there is need for a remote, unattended, long-livedsmall power source that is relatively impervious to conditions andhazards of its environment. Such uses include earth-based ones such asnavigational aids in remote areas, communication relay stations, forestwarning equipment, ocean cable boosters, and the like. This invention isalso of interest for use in pacemaker heart devices and artificialhearts which are of medical interest at present for prolonging the lifeof individuals with certain cardiac deficienies.

For space missions, particularly manned ones, shielding requirementsagainst radiation contribute significantly to the overall weight of thespace vehicle. Astronauts present in the vehicle would require shieldingnot only from external radiation but also from the radiation emitted bythe isotopic power unit itself.

In general, isotopes which are alpha-particle emitters are preferred forfuel use in manned mission space flights because they are relativelyeasy to shield against, alpha radiation being the least penetrating ofall. Exemplary of such a suitable isotopic fuel is plutonium-238.However, the alpha-particle-emitting isotopes are not ordinarily usableas fuel in elemental form, but are present in the form of theircompounds, alloys and mixtures so as to provide an isotopic fuel withsuitable properties with respect to melting point, hardness, ease offabrication and handling, and other related physical and metallurgicalcharacteristics.

While alpha-particle emission per se requires but minimal shielding,secondary radiation resulting from interaction of the primary alphaparticle with material in the immediate vicinity of the isotope emitteraccounts for a significant increase in shielding requirements. The mostimportant secondary source of radiation requiring shielding arises fromthe alpha-neutron reaction in which an element is transmuted byabsorption of an alpha particle, a neutron leaving the excited nucleus.

SUMMARY OF THE INVENTION It is an object of this invention to provide analphaparticle emitter and a method for preparing it whereby thereresults minimal secondary neutron generation.

In accordance with this invention a novel fuel material is providedhaving utility in a radioisotope generator which includes a fuel capsuleand may include a radiation shield in cooperative relation therewithwherein the radioisotope fuel material is an alpha-particleemittingradioactive isotope combined with electronegative components whichessentially have a threshold for the alpha-neutron reaction greater thanthe maximum energy of the emitted alpha particles. Thereby secondaryneutron generation resulting from an alphaneutron reaction issubstantially reduced, with a corresponding reduction inneutron-shielding requirements. While the radioisotope fuel provided bythis invention would not necessarily require a formal shield againstneutrons for use in a pacemaker heart device, the reduction of secondaryneutron emission would still be of considerable importance.

Preferred as radioisotope fuel materials for use in the practice of thisinvention are alpha-emitting radioisotopes of the actinide seriescombined in molecular form with particular low atomic numberelectronegative elements, or with selected isotopes of these elements,which have a threshold energy for the alpha-neutron reaction greaterthan the maximum energy of the alpha particles emitted by theradioisotope. Particularly preferred as fuel material is radioactiveplutonium oxide wherein the plutonium consists essentially of theplutonium238 isotope and the oxygen consists essentially of theoxygen-16 isotope substantially free of, or with only trace amounts of,the oxygen-l7 and oxygen- 18 isotopes. Ordinarily, plutonium-238 oxidewill emit a primary neutron by spontaneous fission and about 15 times asmany secondary neutrons by an alpha-neutron reaction with naturaloxygen. Thus complete elimination of secondary neutrons will reduceoverall neutron emission by a factor of 16.

BRIEF DESCRIPTION OF THE DRAWING For a more complete understanding ofthe invention, reference is made to the sole FIGURE of the drawingshowing a perspective view, partly in section, of an embodiment of aradioisotope generator suitable for use in practice of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawing, whichis intended as illustrative and not restrictive of the presentinvention, a simplified view of a radioisotope generator 1 is shown fromwhich its principal components may be seen. An outer shell 2, usually inthe form of a thin cylindrical can of metal, protects the internalcomponents from contamination and may serve as a heat radiator whererequired. A radiation shield 3 is required to provide safetyrequirements in handling the generator during launch and followingimpact and also to protect astronauts present in the space vehicleduring manned space flights. These shields are of high density andprevent the emission of primary and secondary radiation. Exemplary ofsuch shields are lead, depleted uranium, and cast iron. Low densityneutron shields such as lithium hydride are also required. These shieldsall contribute considerably to the weight of the radioisotope generator.This increase in shielding weight is a major disadvantage for spacemissions. Where the minimizing of shielding weight is a primaryconsideration, it is preferred to use radioisotopes which arealpha-radiation emitters, since alpha radiation is the least penetratingof all and requires minimal shielding provided no secondary radiation ofa penetrating nature occurs.

An energy converter section 4 is used to transform part of the isotopedecay heat into electricity. This may consist of an array ofthermoelectric elements or thermionic converters. At the heart of thegenerator is the energy source, shown as a fuel capsule 5 in which aradioisotopic fuel material 6 is enclosed by a capsule wall 7.

The radioisotope generator may be of any desired shape, cylindricalshapes or spherical shapes being more common. In one type of assembly,the energy converters are placed around a space reserved for the fuelcapsule. The shield is then wrapped around the converters. The outershell, except for an end left open for fuel insertion, is soldered orwelded around the shield. The fuel capsule is then usually inserted byremote control for safety reasons, and the last piece of the outer shellis then sealed in place. Almost all of the nuclear particles emitted bythe decaying radioisotopic fuel are absorbed inside the fuel capable.During the absorption process, the fast nuclear particles collide withthe atoms in the fuel capsule, causing them to move more violently andthus raise the capsule temperature. The kinetic energy of the particlesis thereby converted to heat. Generally about 5 to 10 percent of thetotal heat flow, shown by directional arrows, is converted intoelectricity. The remaining heat energy produced by the fuel flows intothe outer shell from where it is radiated or conducted to a surroundingenvironment.

Because of the physical and metallurgical requirements for the fuelmaterial, an alpha-particle-emitting isotope cannot ordinarily be usedin the pure elemental form. For example, Pu has a melting point rangingfrom 120C. to 640C, depending upon the particular crystalline structure.Whereas, in the form of PuC its melting point is 1654C.; PuO melts at2282C PuN 2450C. Similarly, high melting points are shown for ThO 3050C, ThC 2655 0., U02 2500 0., UN 2630C. Thus the isotopic materials willordinarily and preferably be used in the form of their molecularcompounds, alloys, or non-stoichiometric mixtures.

It is an essential feature of this invention that secondary neutrongeneration resulting from an alphaneutron reaction is substantiallyreduced by selecting the electronegative component that is combined withthe alpha-particle-emitting radioactive isotope to have a threshold forthe alpha-neutron reaction that is greater than the maximum energy ofthe emitted alpha particles. Generally a threshold above about 5.6 Mevis required, this threshold value varying somewhat depending upon theparticular alpha-particle-emitting source.

The selection of the particular alpha-particleemitting radioactiveisotope and its combining component will be determined by many factors.As mentioned, metallurgical and physical properties play a primary role.Also of importance is compatibility of the fuel material with claddingmaterials, as well as its chemical stability, availability and cost.Where a space mission of brief duration is contemplated, an isotope suchas polonium-ZlO which has a half-life of 138 days may be suitable;whereas for a Mars mission, which would require approximately two years,a longer-lived isotope, such as plutonium-238, which has a half-life ofyears, would be required. Also the choice of isotope would be governedin part by the relative freedom of the alpha-particle-emitting isotopefrom other primary radiation emission such as gamma rays and neutrons.

The alpha-particle-emitting radioactive isotopes of the actinide seriesare generally preferred in the practice of this invention. Exemplary ofsuitable alpha-partic1e-emitting isotopes are: Pu Th U Cm Cm and Am, andalso P0 Pu is advantageous for use as an alpha-particle-emittingradioactive source because of its long half-life and the relativelysmall amounts of gamma rays and neutrons which are also primarilyemitted. Thus shielding requirements are considerably minimized.Examples of electronegative components with which the electropositivealpha-particle-emitting isotopes may be combined, either as molecularcompounds, alloys, or non-stoichiometric mixtures, are oxides, carbides,nitrides, silicides, and oxychlorides. Preferred fuel materials for thepractice of this invention are the oxides, carbides, nitrides, silicidesand oxychlorides of the alpha-particle-emitting radioisotopes of theactinide series, e.g., Pu Th U Cm Cm and Am as well as the oxide,nitride, and oxychloride of P0 In Table I is shown the threshold energyfor the alpha-neutron reaction for the relatively light weight naturalisotopes. The relative abundance of these natural isotopes is alsoshown.

TABLE 1 Threshold Energies for Alpha-Neutron Reactlons in NaturalIsotopes from Li to Ni Threshold Energy for Isotope Abundance (01,71)Reaction (Mev) Li 7. 4.85 Li 92.6 5.25 Be 0 (0 [305.) B 18.8 0 (Q pos.)B 81.2 0.22 (3' 98.9 1 1.25 C 1.1 0(0 pos.) N 99.62 6.1 N 0.38 8.15 0'99.76 15.2 0" 0.04 O (Q pos.) 0' 0.20 0.86 F 100.0 2.35 Ne 90.52 6.7 Ne0.27 0 (Q pos.) N6 9.21 8.52 Na 100.0 3.47 Mg 78.6 8.37 Mg 10.1 0(Qpos.) M 11.3 0 (Q POS.) Al 100.0 3.05 Si 92.3 8.25 Si 4.7 1.72 Si 3.03.95 P 100.0 6.58 :12 95.1 9.77 S 0.74 0.75 s" 4.2 5.06 5" 0.0136 3.40C1 75.4 6.52 C1 24.6 4.28 A 99.632 4.28 K 93.1 7.77 K 6.9 3.72 Ca" 96.97No data Ca 0.64 6.58 Ca 0.145 0 (0 pos.) Ca 0.0033 0.25 Ca 0.185 0.15 Sc100.0 2.45 Ti 7.94 4.75 Ti 7.75 0.38 Ti 73.45 2.92 Ti 5.52 0 (Q p05.) Ti5.34 1.92 V 0.23 0.20 V 100.0 2.45

TABLE I- Continued cr= 4.49 h 530' Cr 83.78 3.85 Mn 100.0 3.80 Fe 5.815.95 Fe 91.64 543 Fe 2.21 1.4 Fe 0.34 3.84 Co 100.0 5.42 Ni 67.7 10.36Ni 26.2 8.1 Ni 1.2 4.17 Ni 3.7 6.8 Ni 1.2 4.9

' In accordance with the teaching of this invention, the Ielectro-negative component forming the oxides, carbides, nitrides,silicides, and oxychlorides will include a mixture of the individualstable isotopes of oxygen, carbon, nitrogen, silicon, and chlorine,these individual isotopes having different thresholds for thealphaneutron reaction both less than and greater than the maximum energyof the alpha particles emitted by the radioactive isotope. However, thismixture is treated to contain a concentration in greater than itsnaturally occurring abundance of those ones of the individual isotopeswhich have a threshold for the alpha-neutron reaction greater than themaximum energy of the emitted alpha particles, i.e., O, C", N, Si, andC1 Hence, by reacting the electronegative component containing theconcentrated isotope with the electropositive component containing thealpha-particle-emitting radioactive isotope, the formed primaryalpha-particleemitting radioactive fuel material will have substantiallyreduced secondary neutron generation as a result of an alpha-neutronreaction. If the alpha-emitting isotope is used in the form of itscarbide, e.g., uranium carbide, the C isotope would be eliminated bychemical or physical treatment either prior to or subsequent to theformation of the compound so that the carbide would be substantiallyfree of the C isotope and would consist almost exclusively of the Cisotope, e.g., U C. Thereby, since the threshold energy of the C"isotope is l 1.25 Mev (Table I), no alpha-neutron reaction would occurand secondary neutron emission would be substantially reduced oreliminated. Similarly, if plutonium oxide (PuO were used, theradioactive fuel would consist essentially of Pu O since the thresholdenergy of 0 required for the alphaneutron reaction is 15.2 Mev; whereasthe threshold energy of the O and O isotopes is considerably less. Thisradioisotope fuel, Pu 0 is particularly preferred therefore in thepractice of this invention because of its elimination of secondaryneutron generation as well as its desirable chemical and physicalproperties.

Generally, the desired electronegative isotope will be enriched ingreater than its naturally occurring abundance prior to formation of thedesired fuel. Thereby, standard chemical reactions which are well knownin the art can then be used to prepare the oxides, carbides, nitrides,silicides, and oxychlorides of the desired alpha-particle-emittingradioisotopes of the actinide metals, and the oxide, nitride andoxychloride of polonium. The enriched electronegative isotopes ofoxygen, carbon, nitrogen, silicon and chlorine are either available inthe form of directly usable compounds, or can readily be converted tousable compounds by standard chemical reactions.

The stable isotopes of the elements of the electronegative componentthat are utilized for compound formation are not only selected from theclass of oxygen, carbon, nitrogen, silicon, and chlorine isotopes, butalso will have present in greater than its naturally occurring abundancethose individual isotopes of these electronegative elements which have athreshold for the alphaneutron reaction greater than the maximum energyof the emitted alpha particles. Referring to Table l, it is seen thatthe electronegative component would therefore be concentrated in thefollowing isotopes: 0 C, N, Si and Cl.

The following enriched isotopes are available from the isotopesDevelopment Center of Oak Ridge National Laboratory, operated by UnionCarbide Corporation for the U.S. Atomic Energy Commission: carbon-l2 ininventory form as elemental carbon in an isotopic abundance of greaterthan 99.9 percent (natu-. rally occurring abundance 98.892 percent);chlorine- 35 in the form of NaCl in an isotopic abundance of .sfi t n9.8. P res! llrssquidn .a atedance 75.529 percent); silicon-28 in theform of SiO in an isotopic abundance of greater than 98 percent{naturally occurring abundance 9 2. 2 l percent). g

; Oxygen-16 is obtainable in the form of O -enricli e d viater or-enriched oxygen gas. Water enriched in a the oxygen-16 isotope isobtainable from Volklsotopes, Westwood, New Jersey, with a depletedcontent of 0.007 percent 0 and 0.007 pe r c e nt Q the balance 0'.Oxygenl 6 is conveniently obtained in tablishment, Harwell, England. Theprincipal reaction NISOUI) INmln) q ta) Sumln) Other methods areammonia-ammonium ion exchange reactions and distillation of NO.

Various standard chemical reactions may be utilized for preparing thedesired oxides, carbides, nitrides, sili- .cides, and oxychlorides.Americium metal forms the same compounds with electronegative elementsof Group lll to Vll as do the other actinides. The pink sesquioxide Am Oand the black dioxide AmO are known, as well as a number ofnon-stoichiometric oxides. The dioxide may be obtained by ignition ofamericium compounds in oxygen-l6 gas.

Polonium oxide can be prepared by reacting the metal with oxygen-l6 gasat 250C. according to the reaction Po 0 P00 This method for thepreparation of the oxide is reported by M. l-laissinsky in Polonium"MLIVl-l (tr)(1964), Mound Laboratories, Miamisburg, Ohio. Thorium andplutonium oxides can be prepared by the same reaction except at a highertemperature, 700C.

Thorium and plutonium carbides can be prepared by arc-melting the metalswith carbon-l2, which is the form supplied by Oak Ridge. The reactionsare M C MC where M is plutonium or thorium.

The carbides of americium and of curium may be similarly prepared sinceboth metals have been prepared in macco (multigram) amounts. See L. B.Asprey et al The Chemistry of the Actinides Chem. and Eng. News, pages75-91 (July 31, 1967). Since americium and curium from some of theirother reactions are seen to react like true actinides, a straightforwardpreparation of the carbides would be to arc-melt the metal with thecarbon-12 available from Oak Ridge. The reactions are Am +C AmC and Cm+CCmC which take place rapidly above the melting point of the carbide.Another standard procedure is to hydride the metals mix with carbon,press and sinter. The reactions are Am +l-l AmH Ami-l C AmC H 1 and Cm HCmH The preparation of americium nitride has been reported by K. Akimotoin J. Inorg. and Nuclear Chem, Vol. 29, pages 2650-2652 (1967) by thereaction AmH N11,, 53 7- AmN 2.5H Utilizing this reaction the hydridesofamericium and curium may be reacted with ammonia which is supplied 99%enriched in I the nitrogen-15 isotope to form the respective nitrides asfollows:

AmH NH; AmN 2.5H

CmH NH;, CmN 2.5H

Thorium and plutonium nitride can be prepared by reacting the metalswith ammonia which is supplied containing nitrogen-15. The reactions arePu NH PuN 1.511;.

and

7 form in which the enriched silicon-28 isotope is available from OakRidge, the reaction M0 SiO 4C MSi 4COT which is carried out at elevatedtemperature in vacuum can be utilized. Also, the SiO can be converted tothe metal by the reactions sio 4111 sir, 21 1 0 and sin, 4Na Si 4NaF Theproduct silicon-28 can then be arc-melted with the metal actinide M+SiMSi as described in the General Procedure and Monthly Progress Report,ANL 6658 (1962), Argonne National Laboratories, Argonne, Illinois.

The preparation of americium oxychloride is described by G. T. Seaborgand J. J. Katz in The Actinide Elements, New York, McGraw-l-lill, 1954,page 513, utilizing the vapor phase hydrolysis of AmCl Alternatively, toprepare AmOCl and CmOCl, HCl gas is prepared from NaCl, which issupplied by Oak Ridge with enriched C1 by the reaction Nacl H2504 NEH-IHClm Then the reaction AM (in solution) 3(OH) *Am(OH) would be carriedout followed by Am(OH 3l-lCl AmCl 3H O using the enriched l-ICl gas. Thehydrolysis reaction would then be carried out using water prepared fromoxygen-16,

AmCl H O AmOCl 2HCl according to the procedure outlined above. The samereaction would be used to prepare CmOCl.

Plutonium oxychloride can be prepared by the following sequence ofreactions:

7 using HCl enriched in chlorine-35.

PuCl H O PuOCl ZHCI using H O enriched in oxygen-16. In the case ofthorium, the reactions are Th (in soln) 40H- Th(OH) Th(OH) 4HC1 ThCl 4HO The ThCl is then heated with water vapor ThCl H O ThOCl ZHCL to givethe product.

All of the thorium and plutonium compounds are discussed in The ActinideElements, supra.

Polonium oxychloride PoOCl has been referred to in Nuclear ScienceAbstracts, Vol. 19, pages 24524 (1965).

in general, the oxides, carbides and silicides of the actinides are highmelting, or stable to high temperature. ThO melts at -3000C., PuO isstable to 1000C. but apparently converts to a lower oxide above 1000. AmO and Cm O are probably stable to at least 2000C. ThC melts at 2650C.PuC decomposes above 1650C. and AmC and CmC should be stable to at leastthis temperature. PuSi is stable at 1500C. so the other silicides shouldalso be stable to this temperature. The actinide nitrides probably willdecompose above 1000C. The oxychlorides are in general prepared byhydrolysis with water vapor above 600C. so they are stable to at least500C. 1

Maximum benefits in reducing shielding requirements are obtained wherethere is present as combining EXAMPLE 1 Preparation of EnrichedOxygen-16 Enriched oxygen-16 is conveniently prepared by electrolysis ofheavy water, D 0, which is enriched in the heavier oxygen isotopes. Thisheavy water process is followed by hydrogen sulfide exchange and thenfollowed by distillation to yield a product containing 90% D 0. The 90%D 0 is then electrolyzed to produce 99.75% D 0. During electrolysis, thelighter O isotope comes off initially as a gas, the heavier oxygenisotopes concentrating during this production of D 0 by electrolysis.Using such a process H O is obtainable with a depleted content of 0.007%O and 0.007% O, with the balance O Oxygen-l6 is then convenientlyobtained in gaseous form by electrolysis of this 0'- enriched water.

EXAMPLE 2 Preparation of Pu o a. By reaction with Plutonium MetalPlutonium metal is produced on a continuous basis by electrolysis. Theplutonium metal is then heated in 0' gas at a temperature of 400C. toform PuO b. From Plutonium Oxide by Hydrofluorination PuO containingnautral oxygen and prepared by low firing at a temperature below 480C.is converted to PuF by hydrofluorination at 450C. The PuF is reduced toPu metal by reaction with calcium and iodine. The Pu metal istransferred to a closed system and converted to PuO as above described.

0. From Plutonium Oxide by Treatment with Phosgene Plutonium oxidecontaining natural oxygen is heated at 400C. with phosgene to convert itto plutonium trichloride. The chloride is then reduced with calcium andiodine to form plutonium metal which is then converted to PuO byreaction with 0 gas as above described.

d. From Plutonium Trichloride.

The plutonium trichloride is hydrolyzed in the vapor phase with O-enriched water to form PuO The reaction that occurs is as follows:

PuCl 2H O PuO 3HC1+ rl-l EXAMPLE 3 Decreased Neutron Yield fromAlpha-Neutron Reaction Two samples of radioactive polonium-2l0 arecompared. The samples are obtained as PoCL, deposited on glass. Onesample is dissolved in distilled water containing oxygen of naturalabundance (99.759% 0*, 0.037%0", 0.204%0) and the other in waterenriched in O (.007% O", 0.007% 0, balance 0"). A

small amount of l-lCl is added to both solutions to prevent depositionof Po on the walls of the container. The two solutions are then made tothe same volume in the same size containers so that the ratio of thecount rates observed with a neutron in a fixed geometry is the desiredratio of the neutron production rates. The neutrons from the sourcesolution are thermalized with paraffin so as to permit use of a borontrifluoride neutron detector. This detector is efficient to about 10percent for neutrons entering the sensitive volume. The cross-sectionalarea for the detector is selected to be about 20 cm For a desired countrate of 1,000 counts/min. from the solution of normal water, and a 3-cmthickness of paraffin, about 10 neutrons/min. is required of theradioisotope source. One curie of P0 WlllEQdBEQlrlQ 9 1Z!. ir!-. s nCurie of P021 for each of the samples, approximately 11W) counts/min. isobtained for the solution of normal water and approximately 1,000/30 orabout 30 counts/min. for the D -enriched water (i.e., O depleted waterby a factor of 30). Polonium-2l0 emits alpha particles with an energy of5.3 Mev, compared with 5.5 Mev for Pu According to the data ofSERDIUKOVA et al., Investigation of the (01,11) Reaction on Oxygen,Bull. Acad. Sci. USSR (Phys. ser.) Vol. 21, p. 1018 (1957), the neutronsource from an alpha-neutron reaction on oxygen is proportionalprincipally to the 0 concentration. In the present process, by depletingboth the O and O isotopes, a significant improvement in reducing theemission of secondary neutrons is obtained by using an O -enrichedradioactive alphaemitting oxide.

Where pure P11 0 is used, with the O and O isotopes of oxygen completelyeliminated, the overall neutron source can be reduced by a factor of 16.But even with available O -enriched water, where the water is depletedto a content of 0.007% O and 0.007% 0 there is a reduction of totalneutron emission by a factor of about 10, or a reduction in thesecondary alpha-neutron source by a factor of about 27. Thus in onedesign where an 1 l-inch thick lithium hydride neutron shield for a Puoxide isotope source is required using natural oxygen a reduction shieldthickness to 5 inches is obtained by using the above available O-enriched source. Thus for a space vehicle using as isotope source P110, containing normal oxygen, the 11-inch thick lithium hydride shieldwould weigh about 1000 pounds. With Pu o containing the enriched OS-inch thickshield weighs about 400 0 pounds, resulting in a saving inweight of 600 pounds. This reduction in weight is of course highlysignificant in a space mission.

While the principles of the invention have been described above inconnection with specific materials and processes, it is to be clearlyunderstood that this description is made only by way of example and notas a limitation to the scope of the invention as set forth in theobjects thereof and in the accompanying claims.

I claim:

11. A primary alpha-particle-emitting radiactive fuel material having asubstantially reduced secondary neutron emission by an alpha-neutronreaction comprising a primary alpha-particle-emitting radioactiveisotope combined with a component which includes a mixture of individualisotopes of a selected element containing a concentration therein ingreater than its naturally occurring abundance of those ones of saidindividual isotopes which have a threshold for the alpha-neutronreaction greater than the maximum energy of the alpha particles emittedby said radioactive isotope whereby secondary neutron generation by saidradioactive fuel by an alpha-neutron reaction is substantially reduced.

2. A fuel material according to claim ll selected from the classconsisting of the oxides, carbides, nitrides, silicides, andoxychlorides of plutonium-238, thorium- 228, uranium-232, curium-242,curium-244, and americium-24l and the oxide, nitride, and oxychloride ofpolonium-210, the electronegative component of the radioactive fuelmaterial including a mixture of individual isotopes of an elementselected from the class consisting of oxygen, carbon, nitrogen, silicon,and chlorine which contains a concentration therein in greater than itsnaturally occurring abundance of those ones of said individual isotopeshaving a threshold for the alpha-neutron reaction greater than themaximum energy of the emitted alpha particles.

3. A fuel material according to claim 2 selected from the classconsisting of the oxides of plutonium-238, thorium-228, uranium-232,curium-242, curium-244, americium-24l, and polonium-210 wherein the Oisotope of the oxygen component is present in a concentration therein ingreater than its naturally occurring abundance.

4. A radioactive fuel material according to claim 3 consistingessentially of Pu o, wherein the oxygen component is substantiallyenriched in the O isotope.

5. A radioactive fuel material according to claim 3 consistingessentially of P 0 wherein the oxygen component is substantiallyenriched in the O" isotope.

6. A fuel capsule heat source wherein the kinetic energy of nuclearparticles emitted by a decaying radioisotope fuel contained therein isconverted to thermal energy, the radioactive fuel material being as setforth in claim 1.

7. A fuel capsule heat source wherein the kinetic energy of nuclearparticles emitted by a decaying radioisotope fuel contained therein isconverted to thermal energy, the radioactive fuel material being as setforth in claim 2.

8. A fuel capsule heat source wherein the kinetic energy of nuclearparticles emitted by a decaying radioisotope fuel contained therein isconverted to thermal energy, the radioactive fuel material being as setforth in claim 3.

9. A fuel capsule heat source wherein the kinetic energy of nuclearparticles emitted by a decaying radioisotope fuel contained therein isconverted to thermal energy, the radioactive fuel material being as setforth in claim 4.

10. A fuel capsule heat source wherein the kinetic energy of nuclearparticles emitted by a decaying radioisotope fuel contained therein isconverted to thermal energy, the radioactive fuel material being as setforth in claim 5.

ill. The method of preparing a primary alpha-partiole-emittingradioactive fuel material selected from the class consisting of theoxides, carbides, nitrides, silicides, and oxychlorides of plutonium,thorium, uranium, curium and americium, and the oxides, nitride, andoxychloride of polonium for an isotope fuel generator wherein secondaryneutron generation by an alpha-neutron reaction is substantiallyreduced, comprising providing a first electronegative component forchemical combination with a second electropositive component consistingessentially of a primary alpha-particle-emitting radioactive isotope toform said radioactive fuel material of desired physical andmetallurgical properties,

said first component including a mixture of individual isotopes of anelement selected from the class consisting of oxygen, carbon, nitrogen,silicon, and chlorine which have different thresholds for thealpha-neutron reaction both less than and greater than the maximumenergy of the alpha particles emitted by the radioactive isotope,

said mixture containing a concentration therein in greater than itsnaturally occurring abundance of those ones of said individual isotopeshaving a threshold for the alpha-neutron reaction greater than themaximum energy of the emitted alpha particles,

chemically reacting the concentrated first component with said secondcomponent consisting essentially of a primary alpha-particle-emittingradioactive isotope selected from the class consisting of p zsa 210 zzs232 242 244 and z-ii to form said primary alpha-particle-emittingradioactive fuel material selected from the class consisting of theoxides, nitrides, silicides, and oxychlorides of Fu Th U Cm cm, and Amand the oxide, nitride, and oxychloride of P0 and recovering saidradioactive fuel material having a substantially reduced secondaryneutron emission by an alpha-neutron reaction.

12. The method according to claim 11 wherein an oxygen component havinga natural distribution or the oxygen isotopes oxygen-l6, oxygen-17 andoxygen-18 therein is depleted in its oxygen-l7 and oxygen-l 8 content toform an oxygen component substantially enriched in the oxygen-l6isotope, and the enriched component is reacted with a plutonium-238radioactive isotope to form a radioactive plutonium oxide fuel materialwherein said oxide is substantially enriched in the oxygen-16 isotope.

13. The method according to claim 12 wherein said radioactive plutoniumoxide fuel material consists essentially of Pu O 14. The methodaccording to claim 12 wherein Pu Cl is hydrolyzed in the vapor phasewith 0 enriched water to form a plutonium oxide fuel material consistingessentially of Pu O zg g .UNITED STATES PATENT OFFICE CERTIFICATE OF vCORECTION Patent No. 3, 909% Dated February 5, 1974 Inventor(e) Vahe Keshishian It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

r- Column 3, line 22 "capable" should read --capsule-- Column 8, theformula on line 3 L should read:

Pu(OH) SE01 --0 1 1101 31-1 0 Column 9, line 1 "table" should read--stable-- line 2 "haa" should read --has a--.

Column 10, line 61 "radiactive" should read --radioactive-- Signed andsealed this 16th day of July 1974.

Attest:

MCCOY M. GIBSON, JR. 7 C. MARSHALL DANN Attesting Office-r Commissionerof Patents $323330 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTIONPatent No. 3,790,M+o Dated February 5, 197 4 Inventor(s) Vahe Keshishian It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

Column 3, line 22 "capable" should read --capsule-- Column 8, theformula on line 3 should read:

P I OH HCl PuCl H 0 Column 9, line 1 "table" should read --stable--;line 2 "haa" should read --has a--.

Column 10, line 61 "radiactive" should read -radioactive- 'Signed andsealed this 16th day of July 1974.

(SEAL) Attest:

McCOY M. GIBSON, JR. 2 I C. MARSHALL DANN Attesting Officer Commissionerof Patents

2. A fuel material according to claim 1 selected from the classconsisting of the oxides, carbides, nitrides, silicides, andoxychlorides of plutonium-238, thorium-228, uranium-232, curium-242,curium-244, and americium-241, and the oxide, nitride, and oxychlorideof polonium-210, the electronegative component of the radioactive fuelmaterial including a mixture of individual isotopes of an elementselected from the class consisting of oxygen, carbon, nitrogen, silicon,and chlorine which contains a concentration therein in greater than itsnaturally occurring abundance of those ones of said individual isotopeshaving a threshold for the alpha-neutron reaction greater than themaximum energy of the emitted alpha particles.
 3. A fuel materialaccording to claim 2 selected from the class consisting of the oxides ofplutonium-238, thorium-228, uranium-232, curium-242, curium-244,americium-241, and polonium-210 wherein the O16 isotope of the oxygencomponent is present in a concentration therein in greater than itsnaturally occurring abundance.
 4. A radioactive fuel material accordingto claim 3 consisting essentially of Pu238O2 wherein the oxygencomponent is substantially enriched in the O16 isotope.
 5. A radioactivefuel material according to claim 3 consisting essentially of Po210O2wherein the oxygen component is substantially enriched in the O16isotope.
 6. A fuel capsule heat source wherein the kinetic energy ofnuclear particles emitted by a decaying radioisotope fuel containedtherein is converted to thermal energy, the radioactive fuel materialbeing as set forth in claim
 1. 7. A fuel capsule heat source wherein thekinetic energy of nuclear particles emitted bY a decaying radioisotopefuel contained therein is converted to thermal energy, the radioactivefuel material being as set forth in claim
 2. 8. A fuel capsule heatsource wherein the kinetic energy of nuclear particles emitted by adecaying radioisotope fuel contained therein is converted to thermalenergy, the radioactive fuel material being as set forth in claim
 3. 9.A fuel capsule heat source wherein the kinetic energy of nuclearparticles emitted by a decaying radioisotope fuel contained therein isconverted to thermal energy, the radioactive fuel material being as setforth in claim
 4. 10. A fuel capsule heat source wherein the kineticenergy of nuclear particles emitted by a decaying radioisotope fuelcontained therein is converted to thermal energy, the radioactive fuelmaterial being as set forth in claim
 5. 11. The method of preparing aprimary alpha-particle-emitting radioactive fuel material selected fromthe class consisting of the oxides, carbides, nitrides, silicides, andoxychlorides of plutonium, thorium, uranium, curium and americium, andthe oxides, nitride, and oxychloride of polonium for an isotope fuelgenerator wherein secondary neutron generation by an alpha-neutronreaction is substantially reduced, comprising providing a firstelectronegative component for chemical combination with a secondelectropositive component consisting essentially of a primaryalpha-particle-emitting radioactive isotope to form said radioactivefuel material of desired physical and metallurgical properties, saidfirst component including a mixture of individual isotopes of an elementselected from the class consisting of oxygen, carbon, nitrogen, silicon,and chlorine which have different thresholds for the alpha-neutronreaction both less than and greater than the maximum energy of the alphaparticles emitted by the radioactive isotope, said mixture containing aconcentration therein in greater than its naturally occurring abundanceof those ones of said individual isotopes having a threshold for thealpha-neutron reaction greater than the maximum energy of the emittedalpha particles, chemically reacting the concentrated first componentwith said second component consisting essentially of a primaryalpha-particle-emitting radioactive isotope selected from the classconsisting of Pu238, Po210, Th228, U232, Cm242, Cm244, and Am241 to formsaid primary alpha-particle-emitting radioactive fuel material selectedfrom the class consisting of the oxides, nitrides, silicides, andoxychlorides of Pu238, Th228, U232, Cm242, Cm244, and Am241 and theoxide, nitride, and oxychloride of Po210, and recovering saidradioactive fuel material having a substantially reduced secondaryneutron emission by an alpha-neutron reaction.
 12. The method accordingto claim 11 wherein an oxygen component having a natural distribution orthe oxygen isotopes oxygen-16, oxygen-17 and oxygen-18 therein isdepleted in its oxygen-17 and oxygen-18 content to form an oxygencomponent substantially enriched in the oxygen-16 isotope, and theenriched component is reacted with a plutonium-238 radioactive isotopeto form a radioactive plutonium oxide fuel material wherein said oxideis substantially enriched in the oxygen-16 isotope.
 13. The methodaccording to claim 12 wherein said radioactive plutonium oxide fuelmaterial consists essentially of Pu238O216.
 14. The method according toclaim 12 wherein Pu238Cl3 is hydrolyzed in the vapor phase with O16enriched water to form a plutonium oxide fuel material consistingessentially of Pu238O216.